Method of electrolytically polishing steel



Alg- 25. 1942 R. P. DELAPLACE ETA. 2,294,227

METHOD OF ELECTROLYTICALLY POLISHING STEEL Filed Jan. 10, 1939 PatentedAug. 25, 1942 METHOD F ELECT ING ST ROLYTICALLY POLISH- EEL Ren PaulDelaplace, Paris, and Charles Lon Bchard, Versailles, France, assignorsto Societe Brevets Aero-Mecaniques S. A.,

Geneva,

Switzerland, a society of Switzerland Application January 10, 1939,Serial No. 250,236 In Belgium February 10, 1938 (Cl. 20d-445) 3 Claims.

The present invention relates to methods of electrolytically polishingmetals or alloys, and more especially steel, which methods are of thetype in which use is made, for levelling the metal striae, of anelectrolysis action in a suitable bath containing said metal whichconstitutes the anode (anodic polishing).

The chief object of the present invention is to provide a method of thistype which is better adapted to meet the requirements of practice thanthe methods used up to this time, and especially to improve theefficiency and rapidity of action of the method.

Another object of the invention is to provide a device for carrying outthis method.

According to an important feature of the present invention, We make use,for this polishing, of a solution which while being substantially stablefrom the double point of view of chemical and physical phenomena, iscapable of dissolving the metal in large amounts in such manner as tocause the formation, during electrolysis, of metallic salts which arebut little ionized, therefore bad conductors of electricity, said saltsbeing of the co-called complex" type.

In particular, according to an advantageous embodiment of the presentinvention, we make use of a stable solution of pyrophosphoric acid.

According to 'another feature of the present invention, We haverecourse, for making the electrolyte, to a non-aqueous solution, that isto say a solution obtained by means of a solvent which is not water, inparticular an organic solvent.

Alcohol is particularly advantageous in the case where pyrophosphoricacid is used because it helps in keeping it stable.

Other features of the present invention will result from the followingdetailed description of some specific embodiments thereof.

.Preferred embodiments of the present invention will be hereinafterdescribed, with reference to the accompanying drawing, given merely byway of example, and in which:

Fig. l diagrammatically shows a device for the anodic polishing of apiece of steel (in this example a roller axis) according to the methodof the present invention, said device being made according to thepresent invention;

IFig. 2 shows, also diagrammatcally, but partly, a device of the samekind for the anodic polishing of a roller according to the process ofthe invention, this device being made according to the invention;

Fig. 3 is a diagrammatic view of the operation of the invention.

'Ihe invention is intended for the anodic polishing of a piece of ametal or of a metallic alloy With a view to eliminating the very minuteprojections which can not be removed through mechanical methods(grinding unavoidably produces striae, whereas burnishing, for instancelead burnishing, only temporarily closes these striae) Concerning theprocess considered from a general point of view, it consists inutilizing electrolysis, inthe known manner, in a solution in which thepiece to be treated constitutes the anode.

But, concerning more particularly this solution, it is chosen in suchmanner that, while being sufficiently stable from'e. the physical: and

chemical viewpoints, it produces, with the metal,

, during electrolysis, the formation of at least one salt which is butlittle ionized, therefore a bad conductor of electricity, this saltbeing of the type called, in chemistry, complex salt.

The present invention is based upon the teach'- ing of experience, thephenomena which take place during the electrolysis being probably thosehereinafter described (it being well understood that the followingexplanations must be considered merely as a hypothesis): 1

When an anodic polishing takes place, the piece to be treated A actingas anode (Fig. 3), with striae which, on the drawing, are shown on anenlarged scale with respect to the cathode shown at C, there is atendency to the formation, on the surface of the anode, of a layer aresulting from the electrolytic reactions, the layer in question havinga, resistance different from, and higher than, that of solution b.

The invention is based upon the idea that it is advantageous to increaseas much as possible the resistance `of this layer a, with a view tomaking as high as possible the elliciency of the electrolytic attack ofthe metal and to accelerate the action tending to level up the striae.This action seems to be determined by the, fact that the ratio of theresistances to the flow of the current respectively opposite theprojections and the hollows, that is to say:

in which r is very low in comparison with R1 and R2, is but littledifferent from:

- El Rz the resistance being, at the beginning, very low in comparison.with R2, it follows that there is a levelling action until R1 is equalto Rz.

Therefore our object is to accelerate this levelling and to make it moreleilicient, according to the invention, by increasing the resistance oflayer q, and this in such manner that, owing to a suitable choice of theelectrolyte, said layer tends to be constituted during the electrolyticoperation, by a complex salt, that is to say a salt which is a badconductor of electricity.

It will be readily understood that it will be easy, for someone skilledin the art, to nd various embodiments of the present invention, providedthat this electrolyte is:

a. On the one hand of an electrical resistance r sufiiciently low;

b. Of a viscosity at least equal to that of water (averaging onecentipoise) in order not to interfere with the physical stability of theactive layer of the anode. However, this viscosity must not be too highso that it does not prevent the gaseous disengagements that take placeon the anode and the cathode (for instance, it would be maximum at somepoises, although this is but a mere indication without involving anylimitation).

Among the solutions capable of producing such complex salts, and, in amore general manner, but not exclusively, according to the applicationof the invention which will now be considered, in the case of an anodicpolishing of steel:

We may cite, for instance, oxalic acid, potassium oxalate, perchloricacid, polyphosphoric acid, and so on: it being well understood that thesolution that is considered is nonaqueous, the solvent being organicandpreferably alcohol.

Among the bodies above cited, the derivatives of phosphorus giveparticularly interesting results:

For instance, starting from a solution prepared by saturating withphosphoric anhydride a solu'- tion of ortho-phosphoric acid of 40 B., wefound a considerable improvement of the reflecting power of the surfacethus treated.

The anhydride is added as quickly as it is possible in view of theviolent heating of the solution, in such manner as to obtain a viscoussolution which contains a mixture of orthophosphoric, pyrophosphoric,and polyphosphoric acid.

But, as a matter of fact, the best result seems to be produced bypyrophosphoric acid, which probably produces a complex salt, such asacid pyrophosphate of iron (since iron pyrophosphate is dissolved andthen combines with pyrophosphoric acid).l

For instance, according to the preferred embodiment of the presentinvention, this pyrophosphoric acid is incorporated with a solvent suchas alcohol, in such manner as to obtain a stable solution.

Advantageously, this solution will be concentrated, for instance at therate of 400 grams of pyrophosphoric acid per liter. y

The method according to the present invention being thus defined, itgoes without saying that, concerning the devices or apparatus forcarrying out this method it will be possible to nd various embodimentsin accordance, for inf stance, with the shape and size of the pieces tobe treated. a

These devices or apparatus must be arranged in such manner that theypermit of rapidly evacuatng the calories developed in the solution bythe flow of the current. In order to obtain such a result, according tothe present invention, if use is made, as digrammatically shown by Fig.1,

of a vessel I acting as cathode, this vessel may a good conductor ofheat.

On theA other/hand, concerning the electricy current to be provided inthese apparatus, it must flow through a circuit provided with means forfeeding said current at the desired value, said means including, forinstance, a rheostat Rh.

rThe intensity of the current to be applied at the beginning of theoperation will be such that it produces a quick disengagement of oxygenon the anode. Then this intensity spontaneously rises, and thetemperature, in the case of steel, averages 20 C.

Then the intensity is adjusted in such manner that it tends neither torise nor toA drop spontaneously, this value corresponding to a goodworking of the polishing operation. This value, in the case of thepyrophosphate solution above referred to, is about 30 amperes pervsquare decimeter.

On the other hand, it should be noted that the polishing effect may betemporarily stopped` I by the phenomenon known as passivation of steel.This phenomenon can easily be stopped by contact with a copper rod keptin a very clean state, or by a mechanical system capable ofautomatically producing, from time to time, or when necessary, such acontact.

Concerning the means to be provided, in connection with this apparatus,for holding the piece to be treated, said means depend upon the shape ofthe piece in question.

For instance,l in the case of a roller axis, as shown by Fig. 1, saidpiece is suspended in vessel by means of a steel piece 3, whichconstitutes the electrode support 3, the whole being arranged in suchmanner that the current distribution over the surface to be polished isas uniform as possible.

In the embodiment of Fig. 2, it has been supposed that the piece to bepolished consists of a roller 4, the latter being, for instance placedupon an iron tripod 5. Thecathode can be prolonged by a rod 6 which isplaced at the center of the roller. As to the current inlet to theanode, it can take place by mere contact of an electrode 3 which maypass across an external glass tube 1.

It should be well understood that any other arrangements can beutilized, according to the shape and size of the pieces to be treated.

In any case, whatever be the -embodiment that is adopted, it ispossible, owing to the process according to the present invention, toobtain a polishing which is so perfect that no such result `had beenobtained up to the present time.

For instance, for an axis such as shown by Fig. l, and when use is madeof a solution of pyrophosphoric acid in alcohol, the polishing op'-eration is performed in a period of time of about ten minutes. Then thepiece is removed as rapidly as possib1e,and it is then immersed in anacid solution, so vas to dissolve the traces of ferrie pyrophosphatewhich may have been carried along by the surface. For this purpose,

use is made, for instance, of an aqueous diluted.

and eliminates any powbility oi seizing of the pieces, while itincreases their resistance.

'In a general manner, whilev we have, in the above description,disclosed what we deem to be practical and eillcient embodiments of thepresent invention, it should be well understood that .we do not wish tobe limited thereto as there might be changes niade 'in the arrangement,disposition, and form of the parts without departing from the principleoi' the present invention as comprehended within the scope of theappended claims.

What we V 1. A method oi' anodic polishing of a steel body whichcomprises, arranging the steel body as an anode in an electrolytic bathentially consisting of pyrophosphoric acid in on-aqucous solution in asolvent capable of assuring the stability of the acid during the courseo! the lishing operation, and passing an electric currentthrough saidbath.`

2. In a method according to claim 1, the solvent being alcohol.

l3. In a method according to claim 1, the solvent being alcohol, and theconcentration of the solution being 400 g. of

liter.

' REN PAUL DEmPLAcE.

" CHARLES LEON vBcHARp,

pyrophosphoric acid per i

